An electric power converting apparatus is composed of a converter, a capacitor and an inverter, and supplies electric power to a load such as a motor.
The converter is connected to a power source, and transforms power voltage. When the power source is a system power supply, an AC-DC converter is used. When the power source is a high voltage battery, a DC-DC converter is used. The inverter converts direct current to alternating current, and supplies electric power to a load. The capacitor is connected between the converter and the inverter, and smoothes voltage to be input into the inverter. Generally, an electrolytic capacitor that is suitable for downsizing and cost reductions is used as the capacitor.
In the electric power converting apparatus, direct current flowing from the converter to the capacitor and direct current flowing from the capacitor to the inverter influence each other and accordingly current flowing through the capacitor (hereinafter, referred to as “ripple current”) grows large. When the ripple current is large, the life of the electrolytic capacitor is decreased due to heat generation by an equivalent series resistance. Therefore, the heat generation is reduced by increasing capacity of the capacitor, and this unfortunately impedes downsizing of the electric power converting apparatus.
Patent Literature 1 discloses an invention for detecting ripple current flowing through a capacitor using a resistance connected in series to the capacitor, and reducing an average value of ripple current. However, though the technology disclosed in the Patent Literature 1 is capable of reducing the ripple current on average, reduction of pulsing charging/discharging current is not considered.
In view of this, Patent Literature 2 discloses an invention for reducing pulsing charging/discharging current flowing through a capacitor by setting a frequency of a carrier signal for driving a converter to be twice as high as a frequency of a carrier signal for driving an inverter.